1. Field of the Invention
The present invention relates to color display systems. More specifically, the present invention relates to a single-panel field-sequential color display system for generating high resolution, full color images.
2. Background of the Related Art
There is a need for low cost, high resolution color display systems for use in large screen, high-definition television sets, computer monitors, data projectors and other commercial, industrial, training and entertainment display products.
Full color display is generally implemented using one of five techniques: (1) spatially using color filter arrays; (2) temporarily using sequential color techniques; (3) additively using multiple optical paths; (4) subtractively using stacked display panels; or (5) additively using stacked display panels.
In spatial color systems, each full-color pixel in a full-color display is subdivided into at least three pixels, with one pixel dedicated to each additive primary color. In a cathode ray tube display system, the sub-pixels are implemented with phosphors that are excited by an electron gun, causing them to become luminous. A color filter array consisting of red, green and blue spectral filters is registered to the phosphor pixel array. Similarly, in spatial color display systems that utilize a spatial light modulator, a color filter array is registered to the active pixel elements of the spatial light modulator, such that the transmission or reflection level of each primary color can be locally controlled.
One problem with spatial color systems is that the sub-pixels must be sufficiently small so that they are not individually resolvable by the viewer. The resulting spatial integration of the sub-pixels by the eye yields a perceived full-color image. In addition, because each full-color pixel must be subdivided into three sub-pixels, the spatial light modulators used in spatial color systems require three times the number of pixels than those used in monochrome displays.
In additive split-path color display systems, the three additive primary colors (red, green and blue) are displayed using three separate panels (channels) e.g., three spatial light modulators. The three panels project three different color representations of the same image simultaneously such that the three separate images overlap at an image plane. The three color images xe2x80x9caddxe2x80x9d up to give an accurate full color representation of the image. The main problem with this approach is cost, size and weight. Three separate image sources are required, each with its own set of associated optics. The higher the number of panels, the larger the system. In addition, a complicated combination optics is usually required with this approach.
In a stacked display system, three optical paths are effectively created without wavefront shearing. There are two types of stacked display systems: (1) additive, where each display panel contributes red, green and blue light; and (2) subtractive, where pixels in each display panel subtract red, green or blue light. The term subtractive is appropriate because such systems are analogous to color film. Although all of the light travels along the same physical path, only specific layers of the structure manipulate light in each wavelength band. In practice, a full-color display consists of a stack of three co-registered transmissive display panels, e.g., spatial light modulators, each responsible for independently determining the local transmission of one additive primary color.
Because there is only one physical path, each stage must be made independent of the other using wavelength selective effects. Luminance modulation with a liquid crystal display requires both a polarized input and an effective voltage-controlled analyzing polarizer. Thus, color independent luminance modulation is typically achieved by wavelength selectively controlling the degree of input polarization, and/or the wavelength selectivity of the analyzer.
Compared to split-path display systems, stacked display systems have unique design challenges. In order to obtain high optical throughput, the optical transmission losses of the display panels must be low, the transmission losses of any passive color control elements must be low, and images must be efficiently relayed between display panels. In stacked direct view display systems, there are additional complications associated with color quality and parallax when the display is viewed off-normal.
In field-sequential color display systems, sub-frames are displayed, with each sub-frame comprising the distribution of an additive primary color in a full-color image. In single-panel field-sequential color display systems, a single image source or panel is used. The three additive primary color images are displayed in three separate sub-frames sequentially during one display frame. Display frame rates are typically 60 Hz ({fraction (1/60)} of a second per frame). The three additive primary color images are displayed in sequence at a rate that is three times the frame rate (typically xe2x89xa7180 Hz) or higher so that all three additive primary color images are displayed over the course of one display frame. The eye integrates the sub-frames temporally, yielding a perceived full-color image. This technique is preferable over additive or subtractive three-panel systems in terms of cost and complexity because only one display panel is used.
The main disadvantage of field-sequential color display systems is reduced light output (luminance). This is due to the fact that each separate color image is displayed for only one-third of a frame as compared to a full frame in an additive or subtractive three-panel system. In addition, since the intensity distribution of the image will change according to which color is being displayed, the image source, e.g., spatial light modulator, must be able to respond or switch in {fraction (1/180)} of a second or less as opposed to {fraction (1/60)} of a second in an additive system (all three color image sources remain static for one full frame in a additive split-path system).
In a single-panel field-sequential color display system, the spatial light modulator must be sequentially illuminated with red, green and blue light in synchronism with the driving of the spatial light modulator with red, green and blue image information. This is typically accomplished by sequentially filtering a broad band (white) light source with a color filter, for high brightness applications, or a set of three lasers or three LEDs that can be individually modulated at xe2x89xa7180 Hz.
A color wheel is commonly used as the color filter in single-panel field-sequential color display systems employing a lamp. The color wheel may be divided into thirds, with one-third passing red light, one-third passing green light, and one-third passing blue light. The color wheel is positioned between the light source and the spatially light modulator, and is rotated so that each primary color illuminates the spatial light modulator while the spatial light modulator is driven with the image data for that color.
One disadvantage of using a color wheel is that the color display sequence is fixed and cannot be changed without changing the color wheel. In addition, as the color wheel rotates from one color filter to the next, the spatial light modulator must be blanked for an amount of time that depends on the size of the illumination spot on the color wheel and its rotation rate to avoid color mixing. This blanking time can be longer than the amount of time it takes to load image data into the spatial light modulator, which reduces the display system brightness and hence the optical efficiency. In addition, color wheels require motors, controllers and fans to operate, and contribute to the size, weight, cost, and power consumption of the display system.
Spatial light modulators (SLMs) that can be used with the above-described display systems include transmissive and reflective liquid crystal devices (LCDs) using amorphous silicon thin film transistors and single crystalline silicon systems, digital micro-mirror devices PMDs), optically-addressed SLMs, such as the optically addressable SLMs disclosed in U.S. Pat. No. 4,941,735 to Moddel et al. and in U.S. Pat. No. 5,073,010 to Johnson et al., grating light valves, and others. These systems typically project an image from an array of individual elements, each corresponding to a picture element (pixel) on the displayed image.
One difference between SLMs that use liquid crystals and DMD SLMs is that, in general, liquid crystals must be switched with zero net DC electric field. This is because liquid crystals have ionic impurities that migrate under the influence of an applied electric field. Positive charges migrate to the negative terminal or negative charges are attracted to the positive terminal. This sets up a back electromotive force that causes image xe2x80x9cstickingxe2x80x9d, i.e., the liquid crystal cell stops switching. Because of the purity of active-matrix-compatible nematic liquid crystals, it may be possible to assume that, over time, the statistical variation in pixel voltages will average out to zero net DC. However, this may result in reducing the lifetime and reliability of the spatial light modulator. The safest approach is to DC balance the spatial light modulator.
One method for DC balancing involves applying voltages of equal amplitude and duration, but opposite polarity to the pixels. However, additional time is required to load the opposite voltages (inverse data). For a nematic liquid crystal, the image frame and inverse frame are both viewable as valid image data because the torque on the molecules is a second-order function of the applied electric field. This results in the same optical grey-scale value for both electrical polarities. Ferroelectric and other chiral smectics have a torque that is a first-order function of the applied electric field. Hence, the frame and the inverse (or DC balance) frame are not generally valid frames for viewing.
Spatial light modulators can be classified as analog or binary in nature. DMDs and ferroelectric LCDs are generally binary devices with symmetric response times. Nematic LCDs are generally analog devices. It is possible to display grey-scale or color images with binary spatial light modulators. It is known that when a person views a rapidly cycled sequence of binary images, the person may, if the rate and duration of the images is adjusted accordingly, temporally integrate the sequency of binary images so that they appear to be grey-scale or color images.
A problem with generating grey-scale or color images with binary spatial light modulators is that the binary spatial light modulator must be fast enough to display many binary subframe images on the display such that these subframes are temporally integrated to create grey-scale data. At a display subframe rate of 1/t, the binary spatial light modualtor must be capable of responding in time t. This places a limitation on which binary spatial light modulators can be used. Namely, spatial light modulators generally require response rates at least as great as 1/t Hz (frames per second), to be used for optimizing grey-scale depth in color and display brightness. The temporal integration process requires that t is small, otherwise the display will appear to flicker and will not appear to provide a grey-scale or color image.
Currently, there are a variety of display devices that may be used to output the binary sub-frames. Liquid crystal on silicon (LCOS) devices that have been designed as spatial light modulators have used pixel designs which can be catagorized as being either xe2x80x9cdynamicxe2x80x9d or xe2x80x9cstatic.xe2x80x9d A static pixel design has a memory element at each pixel, which can store the pixel data indefinitely without the need for periodic refresh cycles. This is analogous to SRAM (static random access memory) in computer memory. A dynamic pixel stores data capacitively and requires a periodic refresh to compensate for leakage of the stored charge, analogous to DRAM (dynamic random access memory).
Dynamic pixel and static pixel spatial light modulators share the property that as the array of pixels is addressed in sequence, row-at-a-time, the liquid crystal spatial light modulator begins to update the new image data immediately after the row addresssed. In reasonably high resolution spatial light modulators, such as SXGA display having 1280xc3x971024 pixels, the electronic refresh time is longer than the liquid crystal switching time. For example, if binary data is supplied to the display through thirty-two data wires running at fifty megabits/second, such an array of pixels takes approximately 800 microseconds to update. The liquid crystal can respond to an applied voltage in approximately several hundred microseconds, depending on the liquid crystal cell gap and the liquid crystal fluid properties (e.g., viscosity, Kxx constants). It is valid, therefore, to view the spatial light modulator as being updated in a sweeping motion across this area. True analog displays may use fewer parallel input lines, increasing the time it takes to refresh or write data to the display.
As discussed above, in single-panel field-sequential display systems, precise synchronization between the illuminating color source and the color image data on the spatial light modulator is required. Therefore, the color image data on the spatial light modulator must be simultaneously valid before it can be usefully viewed.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.
The present invention provides single-panel field-sequential full color display systems that are less complex, smaller in size and less costly than prior art additive split-path color systems, while exhibiting higher light output, greater flexibility and greater reliability than prior single-panel field-sequential color display systems.
The display system of the present invention comprises a light source, illumination optics, a color sequencer, a single spatial light modulator and display optics. In a first embodiment, the spatial light modulator comprises a xe2x80x9cframe bufferxe2x80x9d style spatial light modulator, in which a frame buffer pixel circuit is integrated into the spatial light modulator. The frame buffer pixel circuit is used to load an entire frame of data onto the spatial light modulator before displaying that image frame. While image data for a new image frame is being buffered or stored on the pixels, the image data for the previous image frame is displayed. The frame buffer circuit comprises an array of pixel buffers that are coupled to integrated electronics and to the driving electrodes of the spatial light modulator, and is adapted to approximately simultaneously transfer image data from the pixel buffers to the driving electrodes.
In a second embodiment, the color sequencer comprises an opto-electronic color sequencer which allows for the electronic control of the transmission of additive primary colors.
The single-panel field-sequential color display systems of the present invention can be implemented in reflective or transmissive modes, and can be adapted for front projection or rear projection display systems.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.